The background art of the present invention will be described using an excavator as an example.
For example, as shown in FIG. 3, a general excavator comprises a crawler-type base carrier 1, an upper slewing body 2 mounted on the base carrier 1 so as to be capable of being slewed around an axis X perpendicular to the ground, and an excavating attachment 3 attached to the upper slewing body 2. The excavating attachment 3 includes a boom 4 capable of being raised and lowered, an arm 5 attached to a tip of the boom 4, a bucket 6 attached to a tip of the arm 5, and respective cylinders (hydraulic cylinders) for actuating the boom 4, the arm 5, and the bucket 6: namely, a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9.
Japanese Patent Application Laid-open No. 2010-65510 (Patent Document 1) discloses an excavator, such as that described above, further including a hydraulic motor for slewing an upper slewing body, an electric motor connected to the hydraulic motor, and an electric storage device, wherein the electric motor performs a regenerative action, when the slewing is decelerated, to exert a braking force and store the regenerative power in the electric storage device.
This technique, however, can lose a normal regenerative action (a braking action and a power recovery action) in the case of an occurrence of a failure in the electric motor, the electric storage device, or an electric system including a control system that controls them: for example, in the case of a failure of the electric motor which failure prevents a brake torque from being generated or in the case of the state where the electric storage device becomes incapable of recovering regenerative power. The occurrence of such a failure during slewing makes the slewing unable to be stopped and can involve damage of the electric motor or the electric storage device. Thereafter, the slewing is impossible until repairs are made, thus making work impossible.    Patent Document 1: Japanese Patent Application Laid-open No. 2010-65510